This invention relates to an operating circuit for a load in which an oscillator circuit produces a high-frequency power to supply the load. For this purpose, the oscillator circuit itself is supplied with a supply power, for example a rectified mains power. Here, oscillator circuits with voltage-controlled switching elements, for example field-effect transistor half-bridges, are considered specifically. Such operating circuits are principally applied in electronic ballasts for low-pressure gas discharge lamps.
An aspect of such operating circuits which is important for this invention results from the need to initiate the free-running oscillation of the oscillator when operation is started. Frequently, in order to produce the free-running oscillation, coupled controlled transformers, for example, are used to drive the switching elements of the oscillator. However, the positive feedback effect is produced only in the actual oscillation mode and at the beginning has to be produced only, as it were, by an external impetus.
A known solution for a starting circuit which produces this impetus when the supply power is switched on can be found in the German Application DE 195 48 506 A1. Here, after the supply power is switched on, a capacitor is charged via a resistor until the breakdown voltage of a diac is reached. Its breakdown discharges a portion of the charge stored in the capacitor into a drive circuit of a field-effect transistor of a half-bridge oscillator. Further details can be found in the quoted document.